High power, diode laser pumped, solid-state lasers have many potential applications, for example, ceramic or silicon wafer scribing and drilling; diamond cutting and scribing; and laser marking and engraving. Examples of such lasers include TEMoo or low-order-mode (LOM) devices with either continuous wave (CW) or Q-switched output at, for example, 1 μm or at the 2nd, 3rd or 4th harmonic wavelength.
With the rapid development of diode-pumped solid-state lasers during the past two decades, a number of approaches have been investigated with the goal of increasing the overall efficiency of such devices. Pumping schemes using high power fiber coupled diode lasers have been the most popular and efficient approaches for end pumping. Such schemes have shortcomings such as complexity, reliability (pointing and damage), limitations on power scale up, and higher cost.
To achieve high optical-to-optical efficiency, good mode matching is preferred. Mode matching increases the coupling (e.g., overlapping) between the TEMoo mode of the laser resonator and the excited volume in the gain medium within the resonator. In general, there are two basic ways to pump the laser gain medium (crystal): end pumping (longitudinally pumped) and side pumping (transversely pumped). Side pumped lasers have been developed using Nd3+ doped crystals, such as Nd:YLF and Nd:YAG. The side pumping (transversely pumped) scheme generally has better scalability to higher power levels because the pumping radiation from a diode array or a fiber coupled laser diode is directed through a lens or optical system into the laser medium normally perpendicular to the laser cavity mode. With side pumping, strong pumping absorption is preferentially near the side surface of the gain medium while the laser cavity mode is normally located in the interior of the gain medium. For this reason, mode matching can be poor compared to end pumping and can result in low efficiency (typically 15 to 30%). End pumping (longitudinally pumped) remains attractive because of the potential of good mode matching for a highly efficient diode-pumped laser system.
The laser resonator mode cross section is often circular in shape because regular symmetric resonator optics and a circular beam are preferred for many laser applications. The pumping source, for example, a high power diode laser bar, often has an asymmetric beam shape. One example of a pumping source is a 40W laser bar, such as Coherent Product No. 1024355, with a 10×0.001 mm emitting window, 10×35° full divergence angle (FWHM—full width, half maximum) and with M2˜1 in the fast axis direction while more than a thousand in the slow axis direction. In such instances, pumping beam shaping is used. Diode laser beam shaping technologies have been developed using reflective optics or refractive optics. Typically, beam shaping reduces M2 along the slow axis while sacrificing beam quality along the fast axis in order to match M2 in the two directions to form a circular beam. Special optics or special arrangements are used to perform the beam shaping. Focusing or delivery optics are used to guide the beam to pump the gain medium.
Optical fiber, due to its symmetric cross section and flexible characteristic, is widely used for diode laser beam shaping and pumping beam delivery. It has desirable characteristics for end pumping: circular beam shape and quasi-flat top intensity distribution (by multimode fiber or fiber bundle). This can lead to a preferred pumping volume with a parabolic temperature distribution profile. Fiber delivery also has its shortcomings. First, there is system complexity: at the diode laser input side, beam shaping and coupling optics are needed, while at the pumping side, a collimator and focusing optics are needed. Second, loss from the fiber coupling and delivery system is typically 25% or more. Third, reliability is a concern for laser stability because of pointing variation in the pumping beam if the fiber is moved or bent while the laser system is in operation. Fourth, optical damage at the fiber input surface, or mechanical damage due to over bending, twisting or other stress, can cause problems. Fifth, optical damage of the gain medium can produce a limit for pump power scale up at each end of the gain medium. For example, the damage threshold is approximately 19.25 W/mm2 at 1% doping level for one of the most efficient laser crystals, Nd:YVO4.